How to make 3D hollow cuboid objects and cut holes out of it
You can model this object quite easily this with a boolean modifier, and 3 array modifiers as well as a solidify modifier.

As for the actual shape of the holes in the image posted in your question, you can do this with a cube that is subdivided twice, then bevel the edges with Ctrl+B to round the shape and the mousewheel to adjust the cuts. Scale the object on the Z axis to finish it off. 

For the rest of the object:


Add a cube, scale on the x or y axis to the desired length,
and apply scale with Ctrl+AScale. Then delete
the faces at the ends with X in edit mode.
In object mode add a solidify modifier to the cube, and adjust
the thickness to the desired amount.
Place your beveled cube where one of the holes should be. Scale it down accordingly.
Add an array modifier to the small cube with an offset and count
that is  appropriate across the top of the larger cube.
Add a second array modifier to the small cube to move the copies to
the opposite side.
Add a third array modifier to move the first two to base, and apply
all in the order they were added.
Add a boolean modifier to the larger cube set to difference with
the small cube as the target object and apply.

You can delete the beveled cubes, they are no longer needed.